Spanish world No. 1 Rafael Nadal said he was certain that Switzerland's Roger Federer, who many consider the greatest tennis player of all time, "is not finished" even though he has won only one title this year and is on the bubble as far as making the eight-man field at the Barclays ATP World Tour Finals in London.

"I'm sure Federer is not finished. He'll fight to do well in 2014 and will return to playing great tennis," Nadal said.

The 32-year-old Federer has fallen to No. 6 in the world this year after being in the top four since 2003.

Nadal, who has had one of the sport's great rivalries with Federer, said he expected to see the Swiss player back in top form.

The 27-year-old Spanish star has a career record of 21-10 against Federer.

"His tennis is there. He had a good week in Basel," where Federer lost to Argentina's Juan Martin del Potro in the final, and "I have no doubts whatsoever that he will play better than he has this year," Nadal said.

"His talent will allow him to continue being one of the favorites at every tournament," Nadal said.

The Swiss great, winner of a record 17 Grand Slam men's singles titles, has been struggling in the past few months and parted ways with his coach, American Paul Annacone, earlier this month. EFE